In recent years, with the trends of higher speeds and higher image quality in the field of electrophotography, durability, triboelectric stability and fixing ability have been earnestly desired for the toners. In view of these technical problems, polyesters have been widely used because of their high durability and excellent colorant dispersion. However, the triboelectric properties such as triboelectric charges and triboelectric stability are still unsatisfactory, so that further improvements have been desired. Therefore, in order to improve the triboelectric properties of toners containing a polyester as a resin binder, there have been reported toners containing several kinds of charge control agents.
For instance, for the purpose of improving triboelectric stability during durability printing, there has been proposed a developer which contains a polyester, a colorant, a positively chargeable charge control agent and a negatively chargeable charge control agent, wherein the developer has a specified average particle size distribution (see Japanese Patent Laid-Open No. Hei 10-221879). Also, there has been proposed a developer composition for electrophotography, which contains a specified polyester, a positively chargeable charge control agent and a negatively chargeable charge control agent, wherein the amount of the positively chargeable charge control agent used is half, or less, than the amount of the negatively chargeable charge control agent used (Japanese Examined Patent Publication No. Hei 3-63065).
In addition, for the purpose of improving triboelectric charges to thereby reduce toner dropping, there has been proposed a toner which contains a first charge control agent and a second charge control agent, wherein a dicarboxylic acid such as fumaric acid are positively added as the first charge control agent (see Japanese Patent Laid-Open No. 2003-15365).
However, the toners mentioned above have a disadvantage that the toners cannot maintain stable triboelectric charges because they are influenced by their environmental factors such as humidity, temperature and the like. Therefore, in order to improve the environmental stability of triboelectric charges, there has been proposed a technique of using a quaternary ammonium salt as a charge control agent, for instance, a negatively chargeable toner which contains a polyester, a positively chargeable charge control agent and a negatively chargeable charge control agent, wherein a quaternary ammonium salt is contained as the positively chargeable charge control agent (see Japanese Patent Laid-Open No. Hei 2-264970).